It has long been known that ozone is capable of efficiently bleaching paper pulp. Despite numerous environmental, efficiency, and other advantages associated with utilizing ozone to bleach paper pulp, however, at the present time no commercially significant installations exist for ozone bleaching. A significant reason for this is the inability to control the ozone bleaching reaction, which is extraordinarily rapid, and the inability to intimately mix sufficient quantities of ozone in carrier gas with pulp.
In recent years, equipment has been developed, such as the MC.RTM. mixer manufactured and sold by Kamyr, Inc. of Glens Falls, N.Y., which can effect intimate mixing of ozone with paper pulp even though the pulp is at medium consistency (i. e. about 6-15%, preferably about 8-12%). However, even with this equipment, there are difficulties in getting enough ozone into intimate contact with the pulp in a uniform manner, and if the ozone is not uniformly mixed with the pulp the ozone will attack the carbohydrate component of the pulp (in addition to the lignin) in localized area, and thereby significantly degrade the pulp. The problem with getting enough ozone into intimate, uniform, contact with the pulp results since the ozone cannot be added alone, but must be included in a carrier gas. Air and oxygen are the two most common carrier gases, although nitrogen can also be used. Oxygen allows the greatest percentage of ozone to be included therein, but even when oxygen is used as the carrier gas, the percentage of ozone therein is typically only about 3-10%.
Since the ozone is so dilute in the carrier gas, attempts have been made to introduce the ozone under pressure into contact with the carrier gas; however, in the pressure range of 7-8 bar or less, the presence of the carrier gas limits the total amount of ozone which can be effectively added in a single stage. Under the same conditions, it is commercially difficult to perform carrier gas separation at medium consistency after bleaching even utilizing medium consistency pulp handling degassing devices and pumps, such as those sold by Kamyr, Inc. of Glens Falls, N.Y. under the trademark "MC".RTM..
According to the present invention, an apparatus and method are provided which allow effective treatment of medium consistency pulp with ozone. According to the present invention, more ozone can be added than is conventionally possible since degassing of the carrier gas from the pulp is effectively practiced during normal processing, and without the addition of significant energy consuming appliances, only one degassing pump being necessary. Practicing the invention, then, it is possible to add ozone with carrier gas to a fluidizing mixer, so as to intimately and uniformly mix the ozone with medium consistency pulp, at a pressure of about 10-13 bar, yet still achieve effective degassing.
According to one aspect of the present invention, a method is provided for ozone bleaching paper pulp having a consistency of about 6-15% throughout treatment, using a mixer, comprising the following steps: (a) Feeding ozone in a carrier gas, under a pressure substantially greater than 1 bar (preferably at about 10-13 bar), and paper pulp having a consistency of about 6-15%, to the mixer. (b) Effecting intimate and uniform mixing of the pulp and ozone in the mixer. (c) Passing the intimate mixture of ozone and pulp in a first path from the mixer, retaining it in the first path a first time period (e.g., at least about 10-30 sec.) sufficient for at least 90% of the ozone to react with the pulp to effect bleaching thereof. (d) Moving the pulp which has reacted with ozone in a second path, markedly different than the first path, so that separation of gas in the pulp and the pulp occurs, while the gas is maintained under pressure. (e) Removing separated gas from step (d) in a third path, while retaining it under pressure; and (f) removing pulp with gas separated therefrom, from step (d), in a fourth path. Step (c) is preferably practiced by passing the mixture in a vertically upward path, and step (d) is practiced by passing the mixture in a horizontal path where gas and pulp separation begins, and then feeding it into an enlarged diameter retention vessel, to a gas pad (chamber) in the vessel, above the level of paper pulp in the vessel, While the pulp is in the retention vessel, further separation of gas from pulp takes place, allowing the pulp to be pumped from the vessel and simultaneously effectively degassed with a single fluidizing degassing pump.
According to another aspect of the present invention, a method of bleaching paper pulp with ozone utilizing a fluidizing mixer, is provided which comprises the steps of continuously and sequentially: (a) Feeding paper pulp at a consistency of about 8-12% and ozone in a carrier gas at a pressure of about 10-13 bar into the fluidizing mixer. (b) Intimately and uniformly mixing the ozone and carrier gas with the pulp in the mixer. (c) Transporting the intimate mixture of gas and pulp from the mixer while retaining the gas and pulp in contact with each other a sufficient amount of time for about 99% of the ozone to react with the pulp to effect bleaching thereof; and then (d) effecting separation of the carrier gas from the pulp while still at the reaction pressure before further treatment of the pulp.
Apparatus is also provided according to the invention. The invention contemplates ozone bleaching apparatus for paper pulp having medium consistency, comprising the following components: A fluidizing mixer for fluidizing paper pulp at medium consistency while mixing ozone in a carrier gas therewith. An ozone in carrier gas inlet line to the mixer. A pulp inlet line to the mixer. A pulp/ozone mixture outlet line from the mixer. A vertical reaction vessel having a top, a bottom, and a first cross-sectional area, connected to the outlet line at the bottom thereof and transporting pulp mixed with ozone upwardly from the mixer. A generally horizontal tube connected to the top of the vertical reaction vessel, for initiating separation of gas within the pulp from the pulp, and having a second cross-sectional area. A pressurized upright retention vessel having a top and a bottom, and having a third cross-sectional area, significantly greater than the first or second cross-sectional areas, the retention vessel connected to the horizontal tube at a connection point near, but spaced from, the top of the retention vessel. A pulp discharge from the bottom of the retention vessel. A pressurized gas discharge from the top of the retention vessel; and means for maintaining the pulp in the retention vessel at a level below the connection point of the tube to the retention vessel, and maintaining a pad of pressurized gas above the pulp.
Preferably, a curved elbow is provided between the vertical reaction vessel and the generally horizontal tube for connecting them together, and the first and second cross-sectional areas are approximately equal. Also, the reaction vessel and tube and retention vessel are all circular in cross-section, with the diameter of the retention vessel being about twice as great as the diameters of the reaction vessel and tube.
It is a primary object of the present invention to provide for the effective ozone bleaching of medium consistency paper pulp, without requiring substantial energy consuming degassing appliances, and while effectively utilizing oxygen containing carrier gas removed from the pulp after bleaching. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.